you could also put it in a LEGO case if you don't have a 3D printer at hand 😉

What a delightful little case! I love that. It's probably stronger than the 3D printed one, too.

Falonn wrote on 2021-10-13, 12:34:

With the adapter in the 3D printed case, I was less worried about it shorting out on anything, so it's more or less just hanging around. One more benefit of using all of the extension cables is that there is a lot of slack. You can tuck the thing just about anywhere in the case without any trouble. I'm using a CF-to-IDE adapter for storage, so the HDD cage in my case was empty and offered a nice, safe, out-of-the-way place to just leave it sitting around.

Gotcha. Having the PS2 port mounted in an expansion slot is an excellent solution. I probably still have some spare space somewhere in my case!

I've got a weird one. When using a KVM , connecting the adapter to the serial port turns the display mono.
And when I say "connecting the adapter to the serial port" I mean exactly that and only that, just the adapter without PS/2 or power cables 😂

Note that the KVM definitely is the culprit, I've been using the same PC (386, ISA Cirrus Logic 542x) with the same adapter directly connected to the same monitor for almost 2 years.
Level One KVM: https://us.level1.com/collections/kvm/products/kvm-0411

Any ideas/anyone with the same KVM to try?

This does not happen with a "Level One KVM-0212", though it's only a 2 port model.
Is it the same phenomenon if you powercycle the PS2TOSERIAL adapter and connect the mouse?

1541 wrote on 2022-01-27, 13:34:

This does not happen with a "Level One KVM-0212", though it's only a 2 port model.
Is it the same phenomenon if you powercycle the PS2TOSERIAL adapter and connect the mouse?

Ah thank you, I'd assume it's the same KVM. Yes, it's the same. So it appears to be a very special case and not some generic incompatibility.
I can't even imagine what's happening, the mouse driver notices something's wrong too

I investigated a bit more, connecting the adapter confuses indeed the monitor detection and thinks it has a mono monitor connected.
Very strange, I can't imagine what might be causing this, although it's definitely related to the KVM

konc wrote on 2022-01-28, 18:57:

I investigated a bit more, connecting the adapter confuses indeed the monitor detection and thinks it has a mono monitor connected.
Very strange, I can't imagine what might be causing this, although it's definitely related to the KVM

Connecting something to the serial ports adds load to the +12V and/or -12V line. Some VGA cards use these lines to generate the reference voltage for the RAMDAC. Monitor detection often measures the impedance of the monitor by comparing the DAC output voltage to some threshold voltage. So if (for example) the -12V is connected badly to the power supply, any notable load (even a couple of milliamps) could significantly shift the -12V level, and thus influence the monitor impedance detection. If the specific KVM in question already has an excessively high input impedance (a color monitor should have 75 ohm DC resistance on R, G, B), a tiny change on the 12V lines could tip it over the edge of detecting "no 75 ohm termination on red or blue" which indicates a monochrome monitor.

mkarcher wrote on 2022-01-28, 23:37:

Connecting something to the serial ports adds load to the +12V and/or -12V line. Some VGA cards use these lines to generate the reference voltage for the RAMDAC. Monitor detection often measures the impedance of the monitor by comparing the DAC output voltage to some threshold voltage. So if (for example) the -12V is connected badly to the power supply, any notable load (even a couple of milliamps) could significantly shift the -12V level, and thus influence the monitor impedance detection. If the specific KVM in question already has an excessively high input impedance (a color monitor should have 75 ohm DC resistance on R, G, B), a tiny change on the 12V lines could tip it over the edge of detecting "no 75 ohm termination on red or blue" which indicates a monochrome monitor.

Thank you very much for the excellent explanation. It made me understand how the detection mechanism works and dig further.

To begin with, connecting anything to the serial port, even a serial mouse, resulted in mono monitor detection. Meaning that the KVM was already at the border.

But why? Because of the keyboard, an IBM model M. I tried different keyboards with the same KVM and PS2TOSERIAL and there was no mono monitor detected again.
It appears that this particular keyboard brings the KVM to the point where connecting anything else to the serial port messes with the monitor detection.
Directly connected to the PC it's fine, add the KVM and it's not 😀 I guess my best chance now is to try a different VGA card.

Also sorry everyone for posting this in the PS2TOSERIAL thread which apparently had nothing to do with it, but before mkarcher's explanation I was only experiencing the problem when connecting the adapter.